Direct current source for electrolytic metal machining with zener diode surge protection



24, 1965 H. SCHIERHOLT 3,202,599

DIRECT CURRENT SOURCE FOR ELECTROLYTIC METAL MACHINING WITH ZENER DIODE SURGE PROTECTION Filed Sept. 5, 1961 2 Sheets-Sheet 1 I rrvenfarz' Aug. 24, 1965 H. SCHIERHOLT 3,202,599

DIRECT CURRENT SOURCE FOR ELECTROLYTIC METAL MACHINING WITH ZENER DIODE SURGE PROTECTION 2 Sheets-Sheet 2 Filed Sept. 5, 1961 INVENTOR Hams 52H EB 0L BY 41,0 0; mwv ATTORNEYS United States Patent 3 202,599 DlRECT CURRENT SdUlRCE FUR ELEQTRGLYTEC METAL MACHINENG WITH ZENER DIQDE SURGE PRQTECTION Hans Schierhoit, Aachen, Germany, assignor to Deutsche Edelstahlwerlre Aktiengeseilschaft, Krefeld, Germany, and Allgemeine Eleh'tricitats-Gesellschait, Ber-Ein- Grunewald, Germany Filed Sept. 5, 1961, Ser. No. 135,977 Claims priority, appiication Germany, Sept. 7, 1960 1) 34,196 2 (Jlairns. (1. 204-224) The invention relates to a DC. source serving to feed apparatus for electrolytic metal machining, more particularly electrolytic grinding. In such apparatus, an'electrically conducting liquid is placed between a tool electrode, which generally constitutes a metal-bonded diamond grinding wheel, and a workpiece electrode, and the work-piece and the prinding wheel are connected to the two poles of a DC. source. As a result of the electrolytic action of the electric current, the material at the work-piece, which is connected with anode polarity, is removed. In the case of electrolytic grinding with metalbonded diamond wheels, the basic mechanical removal process is greatly increased.

The special advantages of this grinding process have full efiect, however, only if a short-circuit current limiting system in the DC. source prevents thermal damage of the workpiece and grinding wheel as a result of excessive currents in the case of direct metallically conductive contact. Moreover, additional spark or are erosion parallel with the electrolytic process must be prevented. The sparks or arcs not only result in a coarsening of the surface of the workpiece but also greatly increased wear of the tool, particularly the expensive diamond grinding Wheels. The sparks or arcs occur if the voltage between the workpiece and the tool reaches or exceeds the arc limit voltage. This arc limit voltage i dependent upon the material of the workpiece and of the tool and also on the electrolyte used. It is generally about 14 volts. As a result of the inevitable internal inductance of the DC. sources conventionally used for electrolytic grinding voltage peaks exceeding the arc limit voltage at the machining gap between the workpiece and the tool during operation cannot be avoided. The output voltage U is obtained from the following:

where U is the no-load voltage R is the internal resist-ance and L is the internal inductance of the DC. source.

Since the resistance between the workpiece and the wheel generally continually varies during grinding, for example as a result of differences in the distance of the surface being ground from the wheel, or as a result of differences in the condition of the electrolyte, rapid and considerable fluctuations in the output current of the DC. source are inevitable. As a result, voltage peaks far above the arc limit voltage may occur if the working current is reduced or interrupted for a short time, particularly since the no-load voltage has to be made relatively high in view of the required removal capacity.

Control devices have already been disclosed which prevent excessive voltage increase during no-load operation. Control devices are also known'by means of which the voltage between the tool electrode and the workpiece can be reduced if a high-frequency current component occurs in the circuit as a result of are or spark discharges. The occurrence of sparks or arcs can thus be detected by this control device and the voltage of the DC. source 3,202,599 Patented Aug. 24, 1965 can be reduced. This generally obviates any considerable damage but it is a disadvantage that the mean value of the working voltage has to be kept relatively low during the entire machining process, and this is at the expense of the removal capacity.

All devices known hitherto counteract the effects but not the causes of the disturbances.

For this reason, according to an undisclosed proposal it has been suggested that the signal used to control the output voltage of a DO source should be a magnitude corresponding to the first differential quotient of the output current by time, so that the cause of disturbances, namely the internal inductance of the current source, was rendered inoperative. The latter arrangement is admittedly very advantageous as regards its action, particularly in the case of relatively high powers, but is also relatively expensive.

According to the invention, in contradistinction thereto, a DC. source is proposed for electrolytic metal machining, more particularlyelectrolytic grinding, by means of which the arc of spark formation can be effectively suppressed without the mean value of the working voltage having to be kept very much below the arc limit voltage, and which can be embodied with very simple circuitry. The DC. source according to the invention is characterized in that the DC. output is bridged by a Zener diode, the breakdown voltage of which is lower than the arc limit voltage of the workpiece electrode and tool electrode material combination. By a Zener diode is meant a semi-conductor diode which becomes conductive again in the inverse direction if the inverse voltage applied exceeds the so-called breakdown or Zener voltage and which then has a very low internal resistance. The magnitude of the Zener voltage depends on the manufacturing process of the diode and can thus be predetermined.

Thus no voltages appreciably higher than the breakdown voltage-can occur at the Zener diode since this diode acts practically as a short-circuit to such voltages. If the Zener diode breakdown voltage is chosen to be slightly below the arc limit voltage and a diode dimensioned in this way is connected in parallel with the output of the D.C. source and hence also in parallel with the workpiece and the grinding wheel, no voltages higher than the arc limit voltage can occur between the workpiece and the grinding wheel.

If the DC. source no-load voltage is made smaller than or equal to the Zener diode breakdown voltage, the Zener diode need only destroy that proportion of the electrical energy stored in the magnetic field of the internal inductance of the current source which is liberated during the periods of decreasing anode current. This energy, however, is only a small proportion of the total energy used during the grinding process.

However in special cases, in the case of high powers, it may occur that the electrical energy to be absorbed by the Zener diode exceeds its admissable power loss. According to the invention, it is therefore proposed to reduce the power converted in the Zener diode by bridging the Zener diode by means of a series-connected resistance-condenser combination which can absorb part of the magnetic energy liberated during grinding from the DC. source, and to connect the two ends of the resistance additionally to a regulator which in turn acts on a voltage control element arranged in the primary circuit of the DC. source. The control magnitude tapped from the resist-ance contains the first differential quotient of the output current according to time in these conditions.

The control magnitude corresponding to the first differential quotient of the output current according to time may naturally be obtained in some other way, for example by means of a transformer, the primary side of which is connected in the output circuit.

'Using a Zener diode it is thus possible, in the case of very high powers, advantageously to combine other already proposed steps for avoiding sparking or arcing. This'results in a considerablereduction of the cost of the entire plant because security against sparking or arcing is ensured by the Zener diode connected according to the invention and the control circuit components added to relieve the Zener diode can be given much lower value than-if they alone had to prevent arcing, while the grinding capacity remains the same.

The DC. source according to the invention and its ef fect will be explained in detail hereinbelow with refer ence to the drawings.

. FIG. 1 is a schematic circuit diagram of a DC. source incorporating a Zener diode, while FIG. 2 is an-example of the output voltage per unit of ime with and without a Zener diode. V

, FIG. 3 is a schematic circuit diagram of one embodiment of the invention. 7

FIG. 4 is a schematic circuit diagram of a second embodiment of the invention.

What I claim is:

1. In an electro-er-osion machining apparatus an electric operating circuit comprising a direct current source, a tool electrode connected'to said source, a workpiece electrode connected to said source, and a working gap in said circuit between said tool electrode and said workpiece electrode, said gap being filled with an electrolyte,

a non-linear resistor connected across said working gap, sa1d non-linear resistor comprising a Zener diode havinga breakdown voltage which is lower than the critical arcing or sparking voltage across the Working gap between said tool and workpieceelectrodes, a bridging arrangement across said Zener diode, said bridging arrangement In FIG. 1, the output terminals 1 and Z of the D.C.. 7

source with the internal resistance R, and the internal inductance L are conected to the workpiece 4 and the grinding wheel 5, and the two poles of the DC. source are bridged by a Zener diode 6, thebreakdown voltage of which is below the arc limit voltage.

I FIG. 3,'the Zener diode is bridged by a series-connected resistance-condenser combination 7, 8. A control magnitude which corresponds to the first differential quotient comprising a series combination of a resistor and a condenser in parallel with said Zener diode, and "a regulator coupled to said circuit, said regulator responding to the rate of change of the working current in said circuit to regulate the voltage of said direct current source.

2. In an electro-erosion machining apparatus an electric operating circuit comprising a direct current source, a tool electrode connected to said source, a workpiece electrode connected to said source, and a working gap in said circuit between said tool electrodes and said workpiece electrode, said'gap being filled with an electrolyte,

a non-linear resistor connected across said working gap, said non-linear resistor comprising a Zener diode having a breakdown voltage which is lower than the critical arcing or sparking voltage across the working gap between said tool and workpiece electrodes, a bridging arrangement across said Zener diode, said bridging arrangement comprising a series combination of a resistor and acon- .denser in parallel with said Zener diode, and a transformer having its primary winding connected in said cirof the output current according to time and which is j supplied to the regulator 9 is tapped fromthe resistance 7. The regulator 9 which in the simplest case may be a grid-controlled electronic tube acts on the control element 10 of the DO source 3. This additional control reduces the power converted in the Zener diode.

A second embodiment of the invention is shown in FIG. 4. Certain elements of the arrangement of FIG. 4 correspond to those of FIG. 3. These elements are iden tified by like numerals. The principal distinction between the tWo embodiments illustrated isthat in the arlater responding to the rate of change of the working current in said transformer to regulate the voltage of said direct current source.

rangement shown in FIG. 4,'the control magnitude corresponding to the first difieren'tial quotient of the output 'current according to time is coupled to the regulator 9 by a transformer 11. This is accomplished by connect- 0O regulator 9.

References Cited by the Examiner UNITED STATES PATENTS 2,772,232 11/56 Comstocket 3.1.; 204 -143 2,789,25 4/57 Bodle et a1 307-88.5 2,976,462 3/61 Miller 307--88.5 2,979,639 4/61 Williams et al. 4 r 2,983,863 5/61 Keonjian 323-69 2,984,780 5/61 Koletsky 323 79 FOREIGN PATENTS 201,472 3 56 Australia.

GEORGE N. WESTBY, Primary Examiner. JOHN R. SPECK, Examiner. 

1. IN AN ELECTRO-EROSION MACHINING APPARATUS AN ELECTRIC OPERATING CIRCUIT COMPRISING A DIRECT CURRENT SOURCE, A TOOL ELECTRODE CONNECTED TO SAID SOURCE, A WORKPIECE ELECTRODE CONNECTED TO SAID SOURCE, AND A WORKING GAP IN SAID CIRCUIT BETWEEN SAID TOOL ELECTRODE AND SAID WORKPIECE ELECTRODE, SAID GAP BEING FILLED WITH AN ELECTROLYTE, A NON-LINEAR RESISTOR CONNECTED ACROSS SAID WORKING GAP, SAID NON-LINEAR RESISTOR COMPRISING A ZENER DIODE HAVING A BREAKDOWN VOLTAGE WHICH IS LOWER THAN THE CIRTICAL ARCING OR SPARKLING VOLTAGE ACROSS THE WORKING GAP BETWEEN SAID TOOL AND WORKPIECE LECTRODES, A BRIDGING ARRANGEMENT ACROSS SAID ZENER DIODE, SAID BRIDGING ARRANGEMENT COMPRISING A SERIES COMBINATION OF A RESISTOR AND A CONDENSER IN PARALLEL WITH SAID ZENER DIODE, AND A REGULATOR COUPLED TO SAID CIRCUIT, SAID REGULATOR RESPONDING TO THE RATE OF CHANGE OF THE WORKING CURRENT IN SAID CIRCUIT TO REGULATE THE VOLTAGE OF SAID DIRECT CURRENT SOURCE. 